This invention relates to a dual air nozzle apparatus for use in a single tier dryer section of a paper making machine. In particular, the present invention provides an improved apparatus for the threading of a paper web through a single tier dryer section of a paper making machine.
It is well known in the paper making art to use a single tier dryer section to dry a paper web which emerges from the paper press section of a paper making machine. An example of a single tier dryer section is disclosed in U.S. Pat. No. 5,062,216 entitled "SINGLE TIER MULTI-CYLINDER PAPER DRYER" which issued to Hannigan on Nov. 5, 1991, and is assigned to the assignee of the subject invention. The single tier dryer section disclosed in U.S. Pat. No. 5,062,216 includes a plurality of separate drying stages, with each separate stage including at least two steam dryers, and with a vacuum transfer roll interposed between adjacent dryers. Generally, a single tier dryer section includes a plurality of alternate top and bottom felted dryer stages which alternatively dry opposite sides of the web as the web progresses through the single tier dryer section.
In a single tier dryer arrangement there is provided a dryer felt which supports the paper web in each dryer stage. The dryer felt and paper web in each dryer stage extend continuously and alternatively around each dryer and vacuum transfer roll such that the dryer felt is disposed between the paper web and each vacuum transfer roll. A partial vacuum is applied to each vacuum roll as the web and dryer felt traverse around the vacuum roll, and the partial vacuum effectively urges and restrains the paper web into close conformity with the dryer felt. This restraint minimizes cross-directional shrinkage and improves the sheet quality and uniformity of the paper web.
As disclosed in U.S. Pat. No. 5,062,216, there may be provided adjacent to each dryer an air nozzle and, in designated instances, a radiant induction heater. The radiant heater may extend across the width of the web and the air nozzle associated with the heater blows hot air warmed by the heater against the web to provide for removal of moisture and water vapor from the paper web. The air nozzle performs an additional function by assisting in the threading process since the hot air blast is directed to blow the web against the felt as the felt leaves a dryer and moves towards the associated vacuum transfer roll. A doctor blade adjacent each dryer is also provided to scrape off any contaminants of the traversing paper web from the dryer roll surface. Failure to remove the contaminants from the dryer roll surface could have a detrimental effect on the quality of the product being produced. This arrangement, along with the vacuum transfer roll, eliminates the threading of the paper web by the use of ropes during the start-up process, thereby reducing maintenance.
The principal advantage presented by the single tier dryer section disclosed in U.S. Pat. No. 5,062,216 is that the open draw between dryers within each dryer stage is eliminated. Open draw is the passing of the paper web unsupported by the felt from one dryer stage to the next stage. Such passing of the paper web in open draw causes the web to be susceptible to folding, wrinkles and tears. Even though the aforementioned single tier dryer apparatus disclosed in U.S. Pat. No. 5,062,216 is an effective single tier dryer section for drying a web, an occasional problem exists in that the web tends to flutter when transferring between each dryer roll and its associated vacuum transfer roll. Such fluttering often leads to threading problems, and may also result in tearing of the web. More specifically, during the threading of the aforementioned single tier dryer section, the tail of the web tends to disassociate from the dryer rolls and wander in the cross-machine direction as it moves down the machine. In particular, the tail has the tendency to disassociate from the felt and subsequently engage the doctor blade or the threading air nozzles adjacent to each dryer roll thereby causing threading problems. Moreover, a substantial amount of paper web residue may accumulate on the air nozzles thereby minimizing the air flow and effectiveness of the air nozzles.
Thus, it is an object of the present invention to overcome the shortcomings of the prior art single tier dryer section for paper making machinery by providing a new and improved apparatus which achieves more effective automatic threading of a paper web through a single tier dryer section.
Another object of the present invention is to provide a new and improved apparatus for drying a paper web in which the paper web is controlled to remain in contact with the dryer felt as the paper web progresses from the dryer roll towards the vacuum roll.
A further object of the present invention is to provide a new and improved apparatus for drying a web of a paper in which the tail and subsequent paper web thereof are urged away from the doctor blade as the paper web progresses from the dryer roll towards the vacuum roll.
Still a further object of the present invention is to provide a new and improved apparatus which facilitates the threading of a paper web in a single tier dryer while being shielded from the build-up of paper web residue.
Yet another object of the present invention is to provide a new and improved apparatus for facilitating the threading of a paper web in a single tier dryer section which is economical and efficient to manufacture and operate.